Gantry cranes are conventionally used in railyards or other types of shipping yards for loading and unloading large containers or other loads to and from railcars, trailers, etc. Such a crane typically has a steel frame including four vertical columns located at comers of the crane. The columns are joined by upper horizontal beams. These beams support a movably suspended lifting mechanism, known as a grappler or spreader, configured to engage, lift and lower a container or load.
The crane runway surface of a shipping yard may be asphalt, concrete, gravel, dirt, or another suitable surface, collectively referred to herein as a "road surface." So that a conventional gantry crane can be driven around on the road surface of a shipping yard, the crane is equipped with four corner-mounted sets of one or more wheels, referred to herein as "roadwheels" (such cranes are generally described herein as "road traveling" cranes). More specifically, each of the roadwheels is mounted in a yoke or truck at the bottom of a respective one of the columns. Typically, at least two of the roadwheels are steerable, and at least two of the roadwheels are motor-driven. The operator can thereby maneuver the crane around the shipping yard as desired for lifting, lowering, and moving containers.
Conventionally, each of the roadwheels has a rigid steel center having a circumferential rim with a rubber off-highway tire mounted concentrically thereon. The rubber tires are compressible under a load. More specifically, the rubber tires deflect against the road surface under the weight of the crane. The amount of deflection increases due to weight of a container lifted by the lifting apparatus. Additionally, the lifting apparatus may be traversed along the horizontal upper beams of the gantry crane, shifting the weight distribution non-uniformly among the respective roadwheels depending on the particular position of the grappler or spreader. Additionally, tire deflection may result in increased tire wear against the road surface.
It is desirable to minimize tire deflection in order to reduce tire wear. It is also desirable to improve steering control of gantry cranes which lift heavy loads.